1. Field of the Invention
The present invention relates to systems and methods for transmitting communication signals, and in particular to systems and methods for transmitting communication signals across high scan angles.
2. Description of the Related Art
Communications satellites are in widespread use and communication systems based upon high-altitude platforms are under development. Such wireless communications systems are used to deliver television and other communications signals to end users.
The primary design constraints for communications satellites and platforms are antenna beam coverage and radiated Radio Frequency (RF) power. These two design constraints are generally paramount in the payload design because they determine which locations on the ground will be able to receive communications service. In addition, the system weight becomes a factor, because launch vehicles and platforms are limited as to how much payload weight can be placed on station.
Further, in high-altitude platform and Low Earth Orbit (LEO) satellite applications it is often necessary to form multiple antenna beams illuminating the ground. Such communication systems require antennas with on-station beam coverage systems capable of altering the beam scan on the ground as well as the beam shape.
Since it is desirable for ground coverage to be evenly distributed among a wide pattern of cells, producing an even pattern on the ground requires a versatile antenna system capable of high scan angles. This presents a difficult problem to antenna system designers.
Three alternative antenna configurations that are generally known in the art are a multi-feed reflector, a single beam phased array and a multibeam phased array. However, each of these configurations has limitations in high-altitude platform and LEO applications.
One configuration known in the art uses a single parabolic reflector with multiple feeds. In order to generate different beam shapes, multiple feeds are combined using a complex beam forming network. Hence a very large number of feeds and multiple Beam Forming Networks (BFNs) are required. In addition, a very large number of parabolic reflectors, requiring an enormous physical envelope, would be needed to apply this configuration in near Earth applications. Due to the wide angular coverage required, each reflector could be used to produce a spot beam over only a very small portion of the overall coverage area.
Another configuration is described by K. K. Chan, et al., A Circularly Polarized Waveguide Array for LEO Satellite Communications, IEEE AP-S International Symposium, June 1999 which is hereby incorporated by reference herein. The proposed system requires a single beam phased array for each beam. While this approach can be used to form different beam shapes, such a system is costly to produce. Furthermore, due to the inherent nature of a phased array, wide-band operation, necessary for simultaneous transmit-receive applications, can be almost impossible to achieve.
A third configuration known in the art uses a multiple-beam phased array. Antenna configurations using multiple-beam phased arrays are inherently more complex and expensive than other configurations. Furthermore, the wide-angle scanning requirement of near Earth applications necessitates the use of very small elements resulting in a large number of elements in the array, thereby increasing the cost and complexity. Alternatively, a plurality of separate phased arrays may be employed, each one operating over a narrow region, however the complexity and expense would be undiminished.
There is therefore a need in the art for a compact, less costly, antenna system capable of simultaneous transmit-receive applications, without the attendant complexity and/or size of prior art systems.
To address the requirements described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses an apparatus and method for transmitting and receiving signals with a multi-beam reflector antenna assembly.
The present invention teaches a multi-beam antenna assembly, comprising a plurality of rings of single beam shaped reflectors, each reflector having its own feed, wherein the plurality of rings are substantially nested or concentric and disposed on separate planes such that the reflectors of adjacent rings are substantially interleaved as viewed from above. In one embodiment, each feed is diplexed to provide both transmit and receive functionality. In one embodiment, the rings are substantially circular in a concentric configuration. Alternate configurations may employ rings of other shapes in a nested configuration.
The present invention also teaches a method of producing multiple antenna beams, comprising generating a plurality of beams from a plurality of single beam feeds, respectively reflecting each beam from a separate reflector of a plurality of single beam reflector rings to a substantially separate coverage area, wherein the reflector rings are substantially nested/concentric and disposed on separate planes such that the reflectors of adjacent rings are substantially interleaved as viewed from above.
The present invention also teaches a communication system having at least one above-ground platform having a multi-beam antenna including a plurality of rings, each ring having a plurality of single beam shaped reflectors, each reflector having its own feed, wherein the rings are substantially nested or concentric and disposed on separate planes such that the shaped reflectors of adjacent rings are substantially interleaved as viewed from above.
The present invention produces a uniform coverage pattern of cells defined as hexagons on the ground from a near Earth station, orbital or otherwise.
The present invention provides an antenna configuration used to generate multiple beams, with the capability of optimizing each of the beams independently for mainlobe and sidelobe performance.
Each cell is covered by a separate feed and reflector combination. Each feed and reflector can also be optimized to provide uniform cell illumination for both transmit and receive functions.
The present invention can be used to optimize wide-band performance in a simple and effective manner. Further, the number of rings can be extended to generate more beams and thus a greater number of ground cells.